1. Field of the Invention
The present invention relates to a projection lens unit for use in a rear projection device and, more specifically, to a projection lens unit which comprises a projection lens system having a reflection mirror for folding back an optical axis of the projection lens system and a frame fixedly holding the projection lens system, and a method of and apparatus for fixing the reflection mirror to the frame of the projection lens unit.
2. Description of Related Art
Rear projection devices have come into widespread as large-sized screen monitors. Such a rear projection device comprises a front monitor screen, a rear reflection mirror disposed behind the front monitor screen and an optical engine for forming an optical image which is projected onto the rear reflection mirror and then onto the front monitor screen from behind. All of the components of the projection device are incorporated within a housing. The optical engine comprises an illumination optical system for uniformizing an illumination light ray from a high intensity light source, a color separating/recombining optical device for separating the illumination light ray into three primary light rays for formation of primary color images of an image generated by an image generator and recombining the primary color images into a single color image and a projection lens system for magnifying the color image and projecting it onto the rear reflection mirror. Specifically, the projection lens unit comprises a frame and a projection lens system mounted in the frame as one integral unit. The projection lens system comprises a first or converging lens group for converging image information carrying light rays incident thereupon from the color separating/recombining optical device, a second or diverging lens group disposed coaxially with the first lens group for diverging the image information carrying light rays and projecting them as an magnified color image onto a rear reflection mirror and a reflection mirror disposed between the first and the second lens group for folding back an optical axis of the projection lens system. As described in Japanese Unexamined Patent Publication No. 2005-352293, the conventional projection lens unit is fixedly installed in the housing with the mirror abutted and bonded at its periphery on, or otherwise fixed to, a mirror holder of the frame.
The positional accuracy of the reflection mirror of the projection lens unit significantly impinges on a performance of the rear projection device. For example, if the reflection mirror is fixed with a low degree of positional accuracy, an image projected on the monitor screen is off centered or accompanied by a trapezoidal distortion. This results in deterioration in image quality. In order to maintain image quality on the monitor screen, it is essential to perform production of frames of the projection lens unit having precise mirror holders under highly controlled accuracy which demands much man power and high production costs.